Power plants which burn carbonaceous fuels, especially soft coal, as a source of energy must scrub the gases from the burning fuel to remove sulfur oxides in order to avoid pollution of the atmosphere. The use of limestone (CaCO.sub.3) as an aqueous slurry has long been employed for the purpose of scrubbing the sulfur oxides from the flue gas. Normally it is employed in towers which contact the gas with the calcium carbonate slurry in a countercurrent manner. The reaction between the sulfur oxides in the gas and the aqueous slurry of calcium carbonate forms a precipitate of calcium sulfite and sulfate, thus removing sulfur from the gas stream.
One of the problems in removing the sulfur oxides by reacting with the calcium carbonate, or other soluble alkaline earth metal salts, is that when the metal sulfate precipitates it forms scale on the contacting surfaces. Thus, it is desirable that the scale be prevented from depositing during the contacting process, but that it subsequently be allowed to precipitate and be separated from the solution. Thus, the solution can be reconstituted and recycled to the process of removing the sulfur oxides from the flue gas without recycling a major portion of the scalant.
In the past, various methods have been tried to inhibit the scale or counteract its effect. Thus, intermittent washing of the contacting surfaces was attempted, but was useful only with low gas volume or low sulfur content of the gas. When chelating or threshold agents were employed, the scale was prevented from forming during the contacting stage, but then could not be easily precipitated in the settling tanks unless the threshold or chelation limits were exceeded. In this the scale would form in both the contacting stage and the settling tanks.
It would be desirable to find a means by which the metal ions could be maintained in solution during the contacting phase to avoid scale formation, but then be permitted to precipitate so that their sulfate salts could be removed.
The present invention provides just such a method. Particular methylenephosphonates have been discovered which will keep the metal ions in solution during the contacting phase and allow precipitation in a subsequent step.